Impairments of Endocrine Functions: Case Studies - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 244

Furthermore, adult male frogs ( Rana esculenta and Rana lessonae ) sampled from agricul-
tural areas presented with lower plasma titers of testosterone (and higher plasma titers
of estrogen) than frogs sampled from “pristine” areas (Mosconi et al. 2005). McDaniel et
al. (2008) also described an inverse relationship between environmental atrazine concen-
trations and plasma testosterone and ketotestosterone. Murphy et al. (2006a) and Hecker
et al. (2004) were not similarly able to correlate reduced testosterone with atrazine expo-
sure in field exposed frogs, but overall, the evidence for reduced androgens following
exposure to atrazine in adult male frogs is persuasive. However, it remains unclear if the
reduced testosterone titers described in adult male frogs bears any relevance to the likely
levels of physiologically active testosterone during gonadal development, and needs to
be examined.
Despite the persuasive link between atrazine exposure and reduced testosterone in
adult frogs in laboratory and field studies, nobody has yet demonstrated increased aro-
matase gene expression (Hecker et al. 2005b; Oka et al. 2008; Kloas et al. 2009b; Langlois
et al. 2010a) or increased aromatase activity (Hecker et al. 2004; Coady et al. 2005; Hecker
et al. 2005a, b; Murphy et al. 2006a; Langlois et al. 2010a) in an amphibian during sexual
differentiation of the gonads as a consequence of exposure to atrazine. A recent study by
Hayes et al. (2010) demonstrated complete sex reversal of four (10% of exposed animals)
African clawed frogs ( X. laevis ) that had been exposed to 2.5 μg L -1 of atrazine through-
out their larval and postmetamorphic development. Two of these phenotypically female
frogs were examined for aromatase gene expression, and indeed, were demonstrated to
express aromatase in their gonads (Hayes et al. 2010). However, this is to be expected in
functionally female frogs, and is not necessarily indicative of the mechanism of emas-
culation during sexual differentiation. Taken as a whole, the evidence for induction of
aromatase as the mechanism of atrazine-induced endocrine disruption in amphibians
is not strong. So, what else could produce an estrogen-like response without involving
aromatase?
Catabolism of testosterone also occurs via two other pathways: one leading to the forma-
tion of 5α-dihydrotestosterone (potent androgen), and a second leading to the formation of
5β-dihydrotestosterone (5β-DHT; nonpotent androgen) (Figure 9.13; Langlois et al. 2010b).
Although the two DHT isomers are chemically similar, they are stereochemically distinct,
and their formation catalyzed by evolutionarily distinct enzymes (Langlois et al. 2010b). In
X. laevis , the activity of 5α-reductase appears to be unaffected by atrazine at a concentra-
tion of 25 μg L -1 (Kloas et al. 2009b). In contrast, Langlois et al. (2010a) reported increased
activity of hepatic 5β-reductase among Gosner stage 42 R. pipiens tadpoles after exposure
to 1.8 μg L -1 of atrazine. Although the role of 5β-DHT during amphibian development
remains unknown, any up-regulation of 5β-reductase as a consequence of exposure to an
environmental contaminant may result in lower levels of biologically active androgens
such as testosterone, thereby resulting in increased prevalence of female and ambiguous
phenotypes. In the same study (Langlois et al. 2010a), exposure to 1.8 μg L -1 of atrazine also
resulted in significantly skewed sex ratios (1:1.4 male/female) and increased expression of
estrogen receptor α ( eralpha ) mRNA in the tadpole brain. Eralpha is activated upon estro-
gen binding, and has been recognized as a biomarker of exposure to estrogenic chemicals
(Lutz et al. 2005).
9.3.6 Conclusions on Amphibians
Two points of view have developed regarding the environmental risk posed by atrazine
for amphibians, and both camps present their cases eloquently (Solomon et al. 2008; Hayes